High-Voltage DC: An Electric Transmission Alternative

As global demand for energy continues to rise, the need to efficiently and economically transport power from various generation sources increases as well.  The present infrastructure found in the United States for transmission and distribution of electric power is primarily an AC system.  However, recent studies have shown that the use of High-Voltage DC (HVDC) power lines can be less expensive, more efficient, and solve problems with long-distance grid connections.

To understand the advantages of AC over DC, a discussion involving electromagnetic theory and the War of Currents[1] must take place.  To summarize, through the effect of mutual inductance[2], AC transformers can “step-up” and “step-down” voltages and current.  This became extremely valuable on the distribution side of the power grid because of the development of efficient AC induction motors.  With loads requiring different voltages and no equivalent device to easily step-up/down DC voltage, AC transmission and distribution became the standard for our power grid [1].

However, on the transmission side, HVDC transmission lines provide numerous advantages over AC transmission lines.  Higher voltages and lower currents are used in power transmission due to the proportionality of heat loss to the square of the current magnitude: P = VI = I2R.   However, transmission lines are not tapped to serve loads; therefore one voltage level is used (usually 138 kV and larger).  As a result, a DC system is a feasible option.  Some key advantages of HVDC transmission systems over traditional three-phase AC systems include:

  • Less capital costs
  • Grid synchronization

HVDC systems only require two conductors (positive and negative), whereas three-phase AC systems require three conductors plus a neutral return wire.  A comparison of HVDC and AC transmission towers is shown below (courtesy of Siemens Global Website).  At a cost standpoint, HVDC systems eventually become more economical once equipment and line losses are taken into account.  Studies have discussed this “break-even distance” of power line construction [3].

DC systems also do not operate at a frequency.  Therefore, DC lines can be used to combine power grids with different frequencies together by means of rectifying and inverting the voltage and current signals from AC to DC, and then back to AC.  This concept was a major concern for European nations.  “Only 2% of all electricity generated in Europe (including the European part of the former USSR) is traded between regional markets. And with the exception of Finland, which imports electricity from Russia, no EU states are involved in direct electricity trade with Commonwealth of Independent States (CIS) countries” [4].  In Europe’s case, HVDC lines could be used to trade electricity between countries operating at different frequencies.  Although, grid synchronization is a key benefit regarding HVDC power lines, additional costs for inverting units would need to be considered.

All in all, I believe HVDC transmission offers numerous advantages to our growing power grid.  With energy demand steadily increasing across all parts of the world, HVDC transmission provides a viable option for integrating sources that are far away from demand and operating at different frequencies.  With the restructuring of our nation’s power infrastructure beginning, I’m sure we will see an increase in the use of HVDC transmission lines.


[1]  http://www.pbs.org/tesla/ll/ll_warcur.html

[2]  http://www.allaboutcircuits.com/vol_2/chpt_9/1.html

[3]  http://www.energy.siemens.com/hq/en/power-transmission/hvdc/hvdc-classic/

[4]  http://www.europeanvoice.com/article/imported/electricity-trade-%E2%80%93-looking-eastwards/46370.aspx


1 Comment

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One response to “High-Voltage DC: An Electric Transmission Alternative

  1. thsummers

    I also think HVDC systems will be an important part of our future electric transmission system. In our reading on “A Solar Grand Plan”, Zweibel et al contended that HVDC would play a crucial role in delivering solar energy over long distances from solar-rich southwestern desert areas to highly populated regions that demand large amounts of electricity, mentioning that DC lines are cheaper to build and take up less land than AC lines. HVDC could also potentially help deliver energy from our “wind corridor” to highly populated regions. Thus, HVDC systems could help to mitigate one of the main disadvantages of integrating solar and wind energy into our electricity system: that the resources are often located very far from where it is needed.

    It is interesting to note that the original electric transmission and distribution systems developed by Thomas Edison in the 1880s were intended to be DC. AC eventually overtook DC as the standard transmission technology due to technical advantages of AC at the time (one key advantage: it was difficult to convert DC between low and high voltages while this could be easily accomplished for AC using transformers). But now we seem to be moving back toward Edison’s original vision, not just by considering HVDC transmission systems but also by moving back toward smaller distributed generation units located close to loads.

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